Accurate real-time measurements of RF voltage and RF current at selected locations in a plasma reactor are highly sought after, in the processing of workpieces such a semiconductor substrate or wafer. The RF frequencies involved in such measurements depend upon the type of plasma reactor, and may lie in the very high frequency (VHF) range from 30 MHz to 300 MHz, for example, or any other RF frequency range. Such measurements are essential for process control, tool maintenance and process design in semiconductor product fabrication of ultra large scale integrated circuits, solar panels, plasma displays, photolithographic masks, and the like. Processes in which such measurements are needed include reactive ion etching of dielectric materials, conductive or semi-conductive materials and organic materials such as photoresist. Other processes where such measurements are needed include plasma-enhanced chemical vapor deposition, plasma-enhanced physical vapor deposition, and the like.
Such measurements may be accomplished using an RF current probe. An RF current probe typically includes a pick-up coil connected across a primary winding. A secondary winding provides an output voltage representative of the measured RF current near the pick-up coil. The RF current probe includes or is coupled to a signal measuring device. The signal measuring device is connected across the secondary winding. Such a signal measuring device may include signal-conditioning or analog-to-digital converter circuits, for example. Alternatively, or in addition, the signal measuring device may include an oscilloscope. The probe cannot be placed inside the plasma reactor chamber without compromising or damaging its components from exposure to plasma during processing. Therefore, permanent location of such an RF current probe is typically confined to locations outside of the chamber or on exterior chamber surfaces.
Alternatively or in addition, such measurements may be accomplished using an RF voltage probe. An RF voltage probe includes a floating electrode serving as a conductive sensor head connected to a passive network of capacitors, that is, a capacitive voltage divider network. The capacitive voltage divider network is connected at an output node to a signal measuring device. Such a signal measuring device may include signal-conditioning or analog-to-digital converter circuits, for example. Alternatively, or in addition, the signal measuring device may include an oscilloscope. The voltage of the sensor head reflects the local RF electric field near the sensor head, as desired. Unfortunately, it is highly sensitive to the load impedance of the signal measuring device and of the signal path (e.g., a cable) connected from the output node to the signal measuring device. In order to avoid distortion of the measured voltage due to the load impedance of the signal path, the measuring device must be placed very close to (e.g., next to) the capacitive voltage divider network, to minimize the signal path length. Typically, the capacitive voltage divider network is sufficiently close to the conductive sensor head so that they separated by less than centimeter. The measuring device and the capacitive voltage divider network typically may be within two centimeters of one another, to minimize the signal path length and thereby minimize the distortion of the voltage on the sensor head. Distortion arises because the scope end of the coaxial cable is best terminated in a 50 Ohm termination resistor to avoid reflection of the RF signal at this end of the cable. This set up renders the input impedance of the coaxial cable so low as to distort the voltage on the sensor head. Therefore, the combination of the voltage probe and the measuring device constitute an assembly that is not separable. Unfortunately, the measuring device adds such bulk to the entire assembly. As a result, the RF voltage probe and assembly (including the measuring device) cannot be placed inside the plasma reactor chamber. Thus, there has seemed to be no way in which to obtain precise accurate RF measurements inside a plasma reactor chamber.